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Nuclear Reactor – Understanding how it works | Physics Elearnin video
Nuclear reactors are the modern day devices extensively used for power generation as the traditional fossil fuels, like coal, are at the breach of extinction. A nuclear reactor is the source of intense heat which is in turn used for generation of power in nuclear power station. Its mechanism is similar to that of a furnace in a steam generator; the steam is used to drive the turbines of the electric generator system.
A nuclear reactor consists of three crucial components: Fuel elements, moderator and control rods.
Fuel elements come usually in the shape of thin rods of about 1cm in diameter and contain fissionable nuclei, like Uranium (235 92U or 238 92U). These rods vary in number according to the size of the reactor, in large power reactor thousands of fuel elements are placed close to each other. This region where these fuel elements are placed is called the reactor core. These fuel elements are normally immersed in water which acts as a moderator.
The objective of a moderator is to slow down the energy neutrons in a nuclear reactor which are produced during the nuclear fission process by the fuel elements. Thermal neutrons, which are neutrons with energy of about 0.04 electron volts, are capable of producing fission reaction with 235 92U. During the fission reaction process, new neutrons are given out which have energies of about 1 MeV. These neutrons of typically escape from participating in another fission process as they are accompanied by enormous energy release. In f -ct, the probability of these neutrons produce another fission reaction is 500 times less than as compared to that of a thermal neutron. This is where moderator is extremely useful. Moderator has the capability to slow down, or in other words moderate, the speed of these high-energy neutrons, so that they can in turn be used for a chain reaction to trigger multiple fission reactions of other 235 92U nucleus.
Commonly, ordinary or heavy water is used as moderator in nuclear reactors because of the deuterons present in them which are capable of slowing the neutron speed. Water molecules in the moderator are useful in slowing down the high-energy neutrons which leave the fuel-element after nuclear fission. These high-energy neutrons collide with water molecules thereby losing out on some energy with every collision and therefore slow down substantially. A new fission reaction can now be triggered using this slow neutron by striking it with the fuel element.
The third and of the most prominent part of a nuclear reactor are the control rods. In order to get a steady output of energy from the nuclear reactor, every single nuclear fission reaction should trigger another fission reaction and ensure the availability of a spare neutron released to trigger the chain reaction. By controlling the number of spare neutrons available at any given time, the rate of the nuclear fission chain reaction can be controlled. This control on the fission reaction can be maintained using the control rods.
The main function of the control rods is to absorb any excess or spare neutron in the moderator in order to prevent any further fission reaction. Usually such control rods are made of Boron or Cadmium. To increase the rate of fission reactions, these rods can be removed from the moderator. A steady output of energy can be thus maintained by inserting or removing the control rods in the nuclear reactor.
Now that we know the components of a nuclear reactor, let us understand the working of a nuclear reactor. It is usually enclosed in a shield made of thick concrete walls. It consists of a reactor core, pump and heat exchanger. The reactor core and pump are in placed in contact with the water, which is usually the heat exchanger used in reactors. Due to the enormous amount of heat released dusing nuclear fission reaction, this surrounding water gets heated up and changes to steam, which is in turn used to turn the turbines. Thus huge heat energy gets converted into electrical energy. Water is continuously flown in and out of the nuclear reactor using the pump.
Thus a nuclear reactor successfully generates nuclear energy from fission reaction.